247. Strobogrammatic Number II

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Difficulty: Medium

A strobogrammatic number is a number that looks the same when rotated 180 degrees (looked at upside down).
Find all strobogrammatic numbers that are of length = n.
For example,Given n = 2, return ["11","69","88","96"]
.
Hint:
Try to use recursion and notice that it should recurse with n - 2 instead of n - 1.

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public class Solution {
    // https://discuss.leetcode.com/topic/20753/ac-clean-java-solution
    public List findStrobogrammatic(int n) {
        return helper(n, n);
    }
    
    private List helper(int n, int m) {
        if (n == 0) return new ArrayList(Arrays.asList(""));
        if (n == 1) return new ArrayList(Arrays.asList("0", "1", "8"));
        
        List list = helper(n - 2, m);
        
        List res = new ArrayList();
        for (int i = 0; i < list.size(); i++) {
            String s = list.get(i);
            if (n != m) res.add("0" + s + "0");
              
            res.add("1" + s + "1");
            res.add("6" + s + "9");
            res.add("8" + s + "8");
            res.add("9" + s + "6");
        }
        return res;
    }
}